1. Field of the Invention
The present invention deals with the field of combustion technology. It relates to a combustion method in accordance with the preamble of claim 1 and to a burner for carrying out the method.
2. Discussion of Background
Catalytic combustion is a method which can be used in gas turbines to increase the stability of the combustion process and to reduce the levels of emission (cf. for example U.S. Pat. No. 6,339,925 B1). Limits on the load which can be applied to materials and on the operating conditions require the catalytic reactors used to convert only part (typically up to 60%) of the total amount of fuel flowing through the burner. Therefore, the gas temperature which results may not be sufficiently increased to thermally stabilize the combustion of the fuel which remains at the outlet of the catalytic reactor (and comprises a homogenous mixture of fuel, O2, N2, CO, CO2, and H2O at temperatures between 600° C. and 950° C.). Consequently, aerodynamic stabilization is required.
One simple solution involves using sudden expansion downstream of the catalytic reactor, with recirculation zones at the ends of the widening bringing about anchoring (cf. for example U.S. Pat. No. 5,626,017). However, this technique only works at relatively high temperatures at the catalytic reactor outlet. However, if greater dynamic stabilization is required, this can be achieved by the formation of highly swirled flows which promote vortex breakdown. U.S. Pat. No. 5,433,596 describes a double-cone burner in accordance with the prior art which brings about such vortex breakdown. A number of other configurations, for example as described in U.S. Pat. No. 5,588,826, likewise achieve this objective. However, a large-volume vortex of this nature requires relatively complex devices which cause considerable pressure drops.
A simplified vortex generator, which is also known as a SEV vortex generator and is distinguished by reduced pressure losses, has been disclosed by U.S. Pat. No. 5,577,378. It has proven suitable for sequential combustion or combustion with afterburning. The action of the device is based on an exhaust-gas temperature at the outlet of the first burner which is above the self-ignition temperature of the fuel injected in the second burner; the combustion chamber for the afterburning is a burner-free space with a number of vortex generators, the purpose of which is to mix the fuel of the second stage with the exhaust gas from the first stage prior to self-ignition. The degree of circulation and the form of the axial velocity profile can be tailored to the specific requirements by suitable selection of the geometric parameters of the vortex generator (length, height, leading angle) and in extreme cases can even lead to a free-standing vortex breakdown, as is sometimes observed in aircraft with delta wings at large leading angles.
The abovementioned U.S. Pat. No. 5,626,017 has described a combustion chamber for a gas turbine with two-stage sequential combustion in which, in the first stage, the fuel/air mixture produced in a mixer is completely burnt in a catalytic reactor. The exhaust gas which emerges from the catalytic reactor is at a relatively high temperature of 800° C. to 1100° C. Vortex generators, as shown for example in FIG. 1 of the present application, are arranged downstream of the outlet of the catalytic reactor. The vortex generators generate a turbulent flow into which fuel is then injected downstream. The exhaust/fuel mixture which forms then self-ignites and forms a flame front which is aerodynamically stabilized by means of a step-like cross-sectional widening in the flow passage. In this case, the vortex generators have the exclusive function of promoting the mixing of exhaust gas and injected fuel. By contrast, the stabilization of the flame front is effected by the widening of the cross section.
The situation is different in the case of a two-stage burner configuration in which the fuel/air mixture is not completely burnt in the first stage, but rather the exhaust gas from the catalytic reactor contains a proportion of unburnt fuel and at the same time has a significantly reduced outlet temperature (e.g. 600° C. to 950° C.). Since in this case no additional fuel has to be injected in the second stage and accordingly also does not have to be mixed with the exhaust gas from the catalytic reactor, in this case the situation is different in terms of flow technology and in particular with regard to the stabilization of the flame front.